Two types of components are used in certain electrical systems. Two or more of a first type must be connected in parallel with each other. Two or more of a second type must be connected in series with each other. One such electrical system is that for controlling flow of gaseous fuel to a burner. An electronic controller provides the operating power to the various components of the burner control system. Burners, particularly larger ones, have a number of operating requirements for safety. For example, startup must proceed according to a prescribed series of steps involving combustion chamber purging and proving pilot flame. During normal combustion, it is important to constantly monitor the output of a flame detector. If flame is not present, then the flow of fuel to the combustion chamber must be immediately stopped to prevent dangerous accumulations of unburned fuel.
Because the potential for harm when fuel flows uncontrollably is so great, it is customary to have two valves in series flow relationship so if one fails in the open position, the other will still be closed to prevent flow of fuel to the combustion chamber. These valves typically have electrical operators or solenoids to open and close them, with the power for operating them provided by the controller. While the valves are in series flow relationship, their operators are in parallel electrical connection, so that the controller can provide operating power at a single source for simultaneously opening and closing them.
In operating a burner, a number of operating conditions must be present for safety. For the burner itself, proper fuel pressure, sufficient combustion air, and presence of flame are necessary. In a case where the burner is used to heat water or generate steam, other conditions must also be present, such as proper water temperature and water pressure. It is customary to sense many of these conditions with safety or limit switches that open if the condition is not within the preset range. All of these safety switches are connected in series to provide power to the controller or to provide power for the valve operators when all are sensing conditions within the preset limits. If any condition is outside its prescribed limit, the associated switch opens, removing power to the valve operators, and causing the fuel valves to close. For example high and low pressure sensor switches sense fuel pressure. If the fuel supply pressure is below the low-pressure limit setting or above the high-pressure limit setting, the associated switch opens.
Proper installation is a concern with these burner control systems. For example, if the safety switch for a particular condition is omitted or by-passed by mistake or is wired in parallel with another switch, the safety test is absent. Where a number of switches are present in a burner installation, it is relatively easy to miswire one or more switches and relatively difficult to detect the miswires. Accordingly, the fewer switches that must be individually wired by the installer the better.
Limiting the number of parts and wiring present in a burner installation saves cost. If it is possible to use identical parts for different purposes in an installation, returns to scale reduce cost of that part. Further, internal point-to-point wiring within a component of the system is time consuming and thus relatively expensive, so replacing this wiring with more standardized wiring reduces cost.
One way to accomplish this is by combining related safety switches and the fuel valve sets in a single package. First of all, this is less costly because a large number of a few identical devices can be made in a factory setting. Secondly, by combining many of these components in the same package, the space required is reduced. Space in many installations is limited, so by combining a number of required or common components in a single package, the installation process is simplified and less likely to have errors. We find that one particularly advantageous package combines fuel pressure safety switches and two fuel valves with their operators. We will call a package containing both a pair of pressure safety switches mounted to sense out of range inlet fuel pressure and a pair of fuel valves in series, each with their own operator, a fuel control module.
We have developed a universal cable assembly that can be used either for connecting components in serial or parallel. This cable assembly has particular advantage for wiring or connecting fuel control modules. Such a cable assembly comprises a (usually) flexible cable with first and second ends, and having at least first and second internal conductors insulated from each other. A first connector plug is mechanically attached to the first end of the cable. The connector plug has at least first, second, and third terminals. By xe2x80x9cterminalxe2x80x9d we mean a conductive element designed to make electrical contact with another conductive element carried on a separate connector plug, and which may be but not necessarily is, electrically connected to a conductor in the cable. The first plug""s first and third terminals are electrically connected with each other and with the first conductor. The first plug""s second terminal is electrically connected with the second conductor.
A second connector plug is mechanically attached to the second end of the cable and also has at least first, second, and third terminals. The first terminal is electrically connected to the first conductor, and the second terminal is electrically connected to the second conductor. A third terminal is not connected to either conductor.
A cable assembly conforming to nothing more than this broad statement of the invention has little use. In a preferred embodiment, each of the first and second plugs includes first through third slots, for respectively receiving first through third connecting pins and within each of which is respectively the first through third terminal. The terminals within the slots are shaped and positioned to make electrical contact with the connecting pin within the slot. Further, this preferred cable assembly has for each of the second plug""s terminals, a projecting connecting pin. Since usually these components will be assembled in a factory environment, the likelihood of proper connection from the component terminals to the plug terminals is quite high.
The slots in the two plugs and the connecting pins of the second plug should all have what we call xe2x80x9cpredetermined polarized geometryxe2x80x9d. By this we mean that all of the pins in the second plug will simultaneously mate with their respective slots of either plug in only one orientation of the pins with respect to the slots and while mated, make electrical connection with the terminals within the slots, first pin in first slot, second pin in second slot, etc. Such a cable assembly can be used either to connect in series two safety switches each having two connecting pins mounted to match the predetermined polarized geometry, or to connect two valve operators in parallel, each operator having two connecting pins mounted to match the predetermined polarized geometry.
The design makes it irrelevant which plug of the assembly is connected to a particular one of the switches or the operators, and which to the other. Of course, the switches and the operators must be properly connected to their connector pin sets for the proper connection of the individual components to each other, and to the controller.
In one embodiment of the invention, the connecting pins of each of the safety switches must connect to the second and third terminals of each plug.
By using a special adapter, two or more of these cable assemblies can be used to connect more than two devices in series. Such an adapter comprises a plug having a first surface having therein first through third slots having a predetermined polarized geometry. The respective one of first through third conductive pins may be inserted into each of these slots. A first terminal is mounted in the first slot to make electrical contact with an inserted connecting pin. A third terminal is mounted in the third slot to make electrical contact with an inserted connecting pin. A second surface of the adapter has at least second and third conductive pins projecting therefrom and has the predetermined polarized geometry of the second and third slots. The second pin is electrically connected to the first terminal and said third pin is electrically connected to the third terminal. This arrangement creates the situation where the two devices connected in series by the first cable assembly appear across the adapter""s second and third pins as a single two-terminal switch or other device.